1. Field of the Invention:
This invention relates to detergent tablets for dishwashing machines which partially dissolve in the pre-rinse cycle and the remainder dissolves in the main-wash cycle, while possessing satisfactory mechanical strength during handling.
Dishwashing in dishwashing machines generally comprises a pre-rinse cycle, a main-wash cycle, one or more intermediate rinse cycles, a clear-rinse cycle and a drying cycle. This applies both to domestic and institutional dishwashing.
Hitherto, it has been standard practice in domestic dishwashing machines, hereinafter referred to as DDWM, to store the detergent in a dispensing compartment which is generally situated in the door of the machine and which opens automatically at the beginning of the main-wash cycle. The initial pre-rinse cycle is completed solely with cold tapwater flowing into the machine.
In institutional dishwashing machines, hereinafter referred to as IDWM, the pre-removal zone corresponds in principle to the pre-rinse cycle of a DDWM. In institutional machine dishwashing in large kitchens, the detergent fed into the main-wash zone is actually used by overflow in the so-called pre-removal zone for the supportive removal of adhering food remains. Although there are also IDWM in which the pre-removal zone is supplied solely with fresh water, a pre-removal zone supplied with detergent solution is more effective than a pre-removal zone supplied solely with fresh water.
An object of the present invention is to apply the action principle of the dishwashing pre-removal zone of IDWM to DDWM. The addition of a detergent to the actual pre-rinse cycle was regarded as one possibility.
In tests carried out with standard DDWM detergents, the effect of this was that, in addition to the usual dispensing of the detergent from the dispensing compartment in the door of the DDWM, more of the detergent had to be introduced into the machine itself. However, it is a well-known problem that flow-deficient regions exist both at the bottom of the machine and in the liquor sump of the machine. As a result, the detergent product can never be adequately dissolved and, on completion of the pre-rinse cycle, has to be pumped off virtually unused.
Scattering detergent into the cutlery basket via the cutlery placed therein is not advisable because irreversible damage can be caused to silver and fine steel.
It has now surprisingly been found that the disadvantages mentioned above do not arise where detergent tablets according to this invention are used. The addition of one or more tablets may be effected for example in an empty part of the cutlery basket or even elsewhere in the machine.
2. Discussion of Related Art:
The use of tablet-form detergents is well documented in the patent literature. Thus, German patent application 16 17 088 describes tablets for dishwashing machines which may be obtained by tabletting a powder-form mixture of sodium silicate having a ratio of Na.sub.2 O to SiO.sub.2 of from 1:3.25 to 2:1 and a water content of from 0 to 20%, complex alkali metal phosphates, active chlorine compounds, low-foaming nonionic surfactants compatible with the active chlorine compounds, fillers such as alkali metal carbonates, chlorides or sulfates, white paraffin oil and tablet binders, and which are said to be storable and transportable.
German patent application 28 57 001 also describes tablets which essentially contain the same constituents but which are said to possess particularly high alkalinity which may be achieved inter alia by the addition of alkali metal hydroxide. However, high alkalinity is unsuitable for the domestic use of the detergents because, unless the detergents are properly handled, they can lead to skin irritation and, in addition, can damage decorative finishes of the dishware.
According to German patent application 33 15 950, it is particularly advantageous, so far as the required mechanical strength of detergent tablets and their high dissolving rate are concerned, not merely to tablet the mixtures of the constituents, but instead initially to prepare a co-granulate from the alkalinereacting constituents and then to tablet the co-granulate thus prepared under high pressure after the addition of further substances and tabletting aids.
In a commercial DDWM, all the tablets were introduced into the dispensing compartment which provided for the addition of powder-form or granular detergents and which is only designed to open automatically on completion of the pre-rinse cycle using cold tapwater. After about 5 to 7 minutes, by which time they have been completely flushed out from the dispenser into the dishwashing liquor by the water, the tablets develop their full activity with increasing water temperature during the 20 to 30 minute long main-wash cycle. When the tablets were introduced, for example through the cutlery basket, they entered the pre-rinse cycle of the machine, but caused increased damage to decorative finishes of the dishware on account of excessive alkalinity and/or dissolved too quickly and/or disintegrated too quickly and sank without dissolving into the liquor sump of the machine. Accordingly, the quantities of detergent available for the mainwash cycle were no longer adequate.
Accordingly, an object of the present invention is to provide a tablet having a broad solubility profile of which at least 10% by weight is dissolved in only the pre-rinse cycle of a DDWM by the cold tapwater flowing in, producing a pH value of at least 10.0 in the wash liquor, and of which at least 65% by weight and preferably at least 70% by weight is available for the main-wash cycle by virtue of its high solubility in warm water.
In the context of the invention, the solubility profile is understood to be the ratio of parts by weight of the tablet dissolved under the conditions of the pre-rinse cycle of a standard DDWM to the tablet as a whole.
Earlier filed German patent application P 35 41 145.7 describes detergent tablets having a uniform composition and a broad solubility profile for dishwashing machines, containing conventional alkaline-reacting components, more especially from the group consisting of alkali metal metasilicates and pentaalkali metal tripolyphosphates, active chlorine compounds and tabletting aids, wherein the alkali metal metasilicates consist of a mixture of sodium metasilicate nonahydrate and anhydrous sodium metasilicate while the penta-alkali metal tripolyphosphate consists of anhydrous pentasodium tripolyphosphate, the ratio by weight of anhydrous sodium metasilicate to the nonahydrate being from 1:0.3 to 1:1.5.
Earlier filed German patent application P 35 41 147.3 relates to multilayer, more especially two-layer, detergent tablets for dishwashing machines containing conventional alkaline-reacting components, more especially from the group consisting of alkali metal metasilicates and penta-alkali metal tripolyphosphates, low-foaming nonionic surfactants, active chlorine compounds and tabletting aids, characterized in that, in a first cold-water-soluble layer, they contain alkali metal metasilicate nonahydrate and penta-alkali metal tripolyphosphate containing from 7 to 22.4% by weight, and preferably from 15 to 18% by weight, water of crystallization in a ratio of from 0:1 to 1:0 and preferably in a ratio of from 0.35:1 to 1:1, based on the anhydrous substances, and low-foaming nonionic surfactants; and, in a second layer dissolving rapidly at increasing temperatures, alkali metal metasilicate and penta-alkali metal tripolyphosphate in a ratio by weight of from 2:1 to 1:2 and preferably from 1:1 to 1.7 : 1, based on anhydrous substances and active chlorine donors.
Finally, earlier filed German patent application P 35 41 146.5 relates to detergent compactates, more especially for dishwashing machines, based on conventional alkaline components, more especially from the group consisting of alkali metal metasilicates and penta-alkali metal tripolyphosphates, and standard additives of the active chlorine compound, surfactant and/or electrolyte type, characterized in that they consist of coldwater-soluble fused masses or tablets combined with substantially cold-water-insoluble tablets or fused masses which are soluble at the increasing water temperatures of the main-wash cycle of domestic dishwashing machines, the fused masses being combined with tablets and tablets with fused masses of different solubility and the cold-water-soluble tablet layers also containing sodium metasilicate nonahydrate.
The problem as stated above was also solved in all three cases mentioned above. The present invention provides a further solution.